A12 Carburetor Vacuum Advance Port No Pressure

[Alex B210]

Hi Datsun gurus,

I am trying to make my 1976 Datsun B210 / 120Y , 4 door sedan to idle and run smoothly because it's running quite rough at idle, has a tendency to stall at stops and red lights and makes quick pops through the exhaust when cold and under load. 

Pretty sure it is running very rich as the old spark plugs were very carbon fouled, and the exhaust pipe's interior is heavy with soot. So I wanted to dial in the mixture on the carby. I hooked a vacuum gauge to the vacuum advance port on the carby as I have seen it's a common way to dial in the mixture screw to obtain the highest vacuum pressure possible, and then richen a bit and leave it at that.

 

I have read that it would be normal for the vacuum to be at 0 at idle on the vacuum port but then you should have vacuum pressure under throttle which I don't. If I rev it, the vacuum still reads as 0. I know my gauge works because if I suck on it, I can get vacuum pressure.

Any ideas as to what might be happening?

 

I don't see, or think there is, a vacuum port on the manifold itself that I could use to dial the mixture. But I am concerned as to why I don't have vacuum on that port on a new carby. I just changed the PCV valve and there is strong vacuum to be felt if I put my finger there. I also did a quick vacuum leak test with some carb cleaner sprayed around the intake/exhaust manifold seals and where the carby mounts to it. I know this is not very recommended as it can erode rubber but it's the only way of testing leaks I had on hands. I did not hear the rpm jump up suggesting I don't have leaks. All the hoses and lines seem in good condition. For the moment I have plugged the vacuum port on the carby because it is not working obviously. So no vacuum advance. I can't think that will help my idling issues!

I have changed lots of parts since I aquired the datto a few months ago here's a list of what I have changed: fuel filter, fuel pump, ignition coil, spark plugs, carby, new ramflo aircleaner insted of the hot scoop, PCV valve, air breather filter on the rocker cover, oil filter and some sexy new wheels.

 

Any help is greatly appreciated!

[datzenmike]

There is no vacuum advance at idle and shouldn't be. Blow some compressed air into the vacuum advance port to clear it. Re-connect the vacuum advance to the distributor. Connect vacuum gauge directly to the intake manifold.

 

Next time the engine is thoroughly cold step on the gas and take the top off the air filter and check that the choke plate is fully closed. Once the engine warms check that the choke opens fully. If the choke is part on it will run rich and give crappy mileage. If the choke is not on when cold you will often get a lean backfire till engine warms up.

 

 Look at the glass on the front of the carburetor. Fuel should be on the mark or close.

 

I hope you kept the old coil, fuel pump, carburetor and the air filter for spares. The replacements are usually inferior to the originals. Plugs are a good idea as long as they are NGK, wiring and cap and rotor are more important. Bad cap rotor and specially wires can allow the spark to jump elsewhere and it may backfire from this.

 

 

[Alex B210]

A reply from the Datsun god himself! I feel honored! Hi from a fellow canadian living abroad in Australia.

 

I will try blowing some compressed air in the carby vacuum advance port like you're suggesting.

I have the fully manual type choke. I do put a bit of choke when I start it but usually turn it off once I start driving. My neighborhood is quite hilly, not unlike Vancouver, and I find that I have basically no acceleration, or extremely laggy one, when I start driving with some choke on. Very embarrassing going up a hill 10km/h with the gas floored and someone behind. It is also the kind of load that tends to cause my exhaust pops. When I installed the choke, I made sure it closed and opened properly but I'll make sure it's still behaving as intended.

 

I have a feeling my float level might be a bit too high but I'd need to be on a proper leveled ground to be sure. I don't see any place on my intake manifold where I could hook my vacuum gauge to dial the mixture. There's just the carby, PCV valve and the head. Also no tachometer in the datto so that does not help.

There's only the PCV valve I could think of, which is new and has obvious vacuum, but then the hole is bigger than a regular vacuum hose so I'm not too sure how I could hook my vacuum gauge there.

 

the dizzy's cap is looking in good condition, no visible crack or wear on it. I'll pop it and take a look at the points and rotor and take some pictures. I have separated the plugs cables with cable separators to keep the sparks from jumping from cable to cable and mess the timing.

 

I am not too sure if I am running rich or lean. It is confusing to me because many symptoms of both cases are similar. I get small pops through the exhaust at higher RPM especially when it is cold and going up hills in general. It usually drives ok at the beginning of a trip but after 15-20 mins it tends to stall at red lights and stops. These are my main issues.

 

Thanks for your advices again!

Alex

 

 

[afracer]

Carb cleaner thru that port should do it. Otherwise, plug that port and see if you can T into a manifold hose for full manifold vacuum to the distributor vacuum advance. You may have to readjust idle afterwards but it'll run better routed this way regardless.